Republican Presidential hopeful Senator Ted Cruz is going on offense against businessman Donald Trump, who has been attacking Cruz over his so-called ‘birther’ issue.

Hugh Hewitt (HH): Now Senator Cruz, I want to turn to the silliest story out there, your eligibility for the presidency. On ABC News this weekend, I said that not a single serious legal scholar had said other than that you were eligible. Immediately, they found a professor at Widener University to write for the Washington Post to argue the point, and Laurence Tribe. You must have given too good of an answer in his class. I think he’s just messing with you, Senator.

Ted Cruz (TC): (laughing) Well, listen, it is not a surprise that liberal Democratic activists, well, Laurence Tribe is a liberal Democratic activist. He is a hard-core Hillary Clinton supporter. And suddenly, Hillary Clinton supporters are rushing out to support Donald Trump’s attacks directed at me. And it starts to make you think, gosh, why are Hillary’s strongest supporters trying to prop up Donald Trump? You know, the last couple of elections, we’ve seen that the Democrats have gotten the candidates they wanted to run against in the general election. And right now, Donald is losing to Hillary by a pretty significant margin in the national polls, and I’m beating Hillary. And so we should expect liberal Democrats to continue echoing Donald’s attacks. I’m not going to engage in the back and forth, the insults. On the merits, the legal issue, is straightforward. A child of a U.S. citizen born abroad is a natural born citizen. And the reason these attacks are happening is because other candidates, especially the Trump campaign, are getting nervous that conservatives are uniting behind our campaign. The campaign is turning more and more into a two person race between Donald and me, and they don’t like the fact that conservatives are coalescing, Evangelicals, libertarians, conservatives are all coalescing behind our campaign. If that keeps happening, if conservatives unite, we win.

HH: Now Senator Cruz, there’s only one clever argument. It’s not persuasive, but there’s only one clever argument about your eligibility. It is that President Kennedy and the Congress of the United States made Winston Churchill an honorary citizen. They did so because he had declared himself a British subject, even though his mother was English. I expect that to show up before long. What’s your response going to be when that shows up?

TC: Oh, listen, the simple fact that this question is governed by American law.

HH: Yup.

TC: And in particular, American law at the time did not pass U.S. citizenship on through the mother. It only passed it on through the father when Winston Churchill was passed. That is not American law today. And so under American law, Churchill was not a citizen at the time. That’s why he was made an honorary citizen. Under American law, I was a citizen the instant I was born. My mother was a citizen at birth. She was born in Wilmington, Delaware. And it is the act of being born that made me a U.S. citizen. I’ve never breathed a breath of air on Earth that I wasn’t an American citizen. I’ve never been naturalized. And when American soldiers are abroad, and they have children, their children are natural born citizens. That’s why John McCain was a natural born citizen, even though he was born in Panama. When American missionaries are abroad, their children are natural born citizens. That’s why George Romney, Mitt’s dad, was a natural born citizen, even though he was born in Mexico. And he ran for president in 1968. And likewise, Barry Goldwater, who was born in Arizona before Arizona was a state, was a natural born citizen, because his parents were citizens. And that’s American law under the Constitution and under federal statute. And as a legal matter, that’s the end of the matter.

Cruz also took a shot at Trump by saying he “embodies New York values,” which would put him out of step with both Iowans for the nation’s first primary and more conservative Republicans as a whole. On Carr’s show:

Campaigning across New Hampshire, where Trump holds a dominant lead, Cruz responded sharply to Trump’s taunts over his White House eligibility. For the past week, and as recently as 24 hours ago, Cruz has tried to brush off Trump’s broadsides as political silliness, but on Tuesday he suggested Trump’s comments reflected his cozy relationship with prominent Democrats.

“I would say Hillary would know well how to identify Democrats. She has been a partisan Democrat herself,” Cruz said on WRKO, when asked about Clinton’s recent statement that Trump used to be a Democrat. “She and Donald know each other well, and I do think it’s interesting that Hillary Clinton’s key supporters are doing everything they can to echo Donald’s attacks on me.”

After months of caution about criticizing Trump’s campaign, Cruz’s recent responses have grown bitterer, according to CNN.

Since the real estate mogul last week began suggesting the Texas senator may not be eligible to be president given his Canadian birth, Cruz has needled Trump’s style of campaigning.

Asked in Decorah, Iowa, how he differs from Trump, Cruz offered that he is campaigning in the “Iowa way” — an aggressive schedule of retail politicking, which Trump has not done to date.

Trump has also raised questions about the faith of Cruz, who identifies as southern Baptist, like his Cuban-born father.

“Not many evangelicals come out of Cuba,” Trump recently quipped.

Cruz countered in his WRKO interview.

“Listen, any time somebody’s attacking your faith, it starts to suggest they’re getting really nervous about what’s happening in the race,” Cruz said.

Trump’s main argument has been that Cruz is constitutionally unable to assume the White House, a position bolstered this week by prominent Harvard Law professor Laurence Tribe, who said he agreed with Trump.

Still, the gloves have not completely come off: While Trump has criticized Cruz’s policy positions on ethanol mandates and portrayed himself as the true leader on border security, Cruz has yet to directly attack the New Yorker’s bona fides on those issues. But Cruz’s comments Tuesday are the latest tension point between the top two Republican candidates in Iowa, where polls show them neck and neck.

“I will say it is more than a little strange to see Donald relying on as authoritative a liberal, left wing, judicial activist Harvard Law professor who is a huge Hillary supporter,” Cruz told reporters in Hudson, New Hampshire, Tuesday. “It starts to make you think, gosh, why are Hillary’s strongest supporters backing Donald Trump?”

Cruz then added that Democrats are eager to see Trump as the GOP nominee and suggested that should raise a red flag.

“You know, the past couple of elections we saw the Democrats thrilled that they got the nominee they wanted to run against in the general election,” he said. “And it seems the Hillary folks are very eager to support Donald Trump and the attacks that are being tossed my direction.”

Cruz and Trump went a long time mostly avoiding one another in their political attacks. That period seems to be over. The head-to-head battle between the two current GOP frontrunners seems to now be fully engaged.